Lasers emit concentrated light via the optical amplification and stimulated emission of electromagnetic radiation, which in turn occurs through an application-specific laser gain medium. The light from ordinary light sources, such as the sun or an incandescent light bulb, is spread over a broad band of wavelengths. In contrast, laser light is usually contained within a very narrow wavelength band, and is often described as being monochromatic. Additionally, the light is emitted in phase and thus is highly coherent. Laser light beams are also collimated rather than spreading out in all directions in the manner of ordinary light. Because laser light is monochromatic, coherent, and collimated, an emitted laser beam may be used to irradiate a very small area and thus achieve very high power densities even with moderate overall power levels. Generally, single-mode/wavelength lasers have higher quality output with lower divergence, more uniform spatial beam profile, and more pure spectral content than multi-mode/wavelength lasers. However, generating high peak power single-mode laser beams is significantly more challenging than generating multi mode laser beams due to a number of non-linear effects occurring in the laser gain media.
A laser cavity or resonator lies at the heart of a laser device. A suitable laser gain medium such as a rare earth active crystal, a gas, or a semiconductor material is enclosed and positioned along the optical axis of the resonator. Mirrors may be disposed a distance apart from each other, with one mirror being a total reflector and the other a partial reflector. As light reflects between the mirrors, the light gains in intensity with each reflected pass through the laser gain medium. Some light escapes through the partial reflector, also referred to as an output coupler, with the escaping light forming the emitted laser beam that ultimately propagates along the optical axis.
The laser gain medium may be a solid, gas, or liquid, with continuous wave (CW) and pulsed lasers commonly used in commercial and scientific research applications. CW lasers in particular produce an uninterrupted beam of light with a stable but relatively low peak output power. Pulsed lasers are able to sustain laser action over brief intervals, even down to pulse lengths of one nanosecond or less. Because pulsed lasers can release significant amounts of stored energy in these pulses, each pulse can have a high peak power level often ranging from several kilowatts to multiple megawatts. In contrast, the peak power of a CW single-mode laser is ordinarily limited to no more than a few hundred watts.
Solid-state lasers use laser gain material in the form of a solid active matrix, such as a ruby crystal or another active element, e.g., neodymium: yttrium-aluminum garnet (“YAG”). Such lasers, while capable of outputting the high peak power levels noted above, are relatively inefficient and usually insufficiently coherent for performing certain precision tasks. A solid-state laser diode is an example of efficient, small, low-power laser devices which can be scaled to high power by combining them into compact multi-diode packages. While the divergence and coherence degrades as the result of the combining, these devices have high utility as a pump sources to excite other laser gain media. The guiding of pump beams at the cladding-to-cladding interface in the hybrid fiber rod is a means to mitigate the high divergence of high power diode pump lasers. Another important type of laser is the fiber laser, with gain media composed of extended lengths of doped optical fiber. Conventional fiber lasers typically have fiber gain lengths of one or more meters, and core diameters on the order of less than 10-20 microns for single-mode operation and around 100 microns for multi-mode operation. Light passing along the extended fiber length is internally amplified and emitted in an efficient and highly coherent manner, but due to fiber structural limitations posed by small core cross-sectional area and long lengths, the resulting high peak power densities of emitted light within the core limits the overall peak power to a correspondingly low level relative to solid-state lasers.
The most highly limiting process in fiber lasers is usually Stimulated Brillouin Scattering (SBS), which can not only severely degrade beam propagation, but can even produce catastrophic material damage to the fiber. The threshold power (Pth,SBS) at which SBS reaches debilitating levels in dielectric materials, including optical fibers and hybrid fiber rods, scales with the quantity d2/L. Thus the small core diameters (d) and long fiber lengths (L) of conventional fiber lasers means that SBS becomes a threat at very low power threshold levels. Much higher power levels can be reached (before SBS becomes a problem) with the larger core diameters and shorter length gain media of the subject hybrid fiber rods.
Currently, there are many Earth-based and planetary mission-based measurements that cannot be performed due to size, mass, power, and thermal concerns from the required laser transmitter. Such missions include ASCENDS (“Active Sensing of CO2 Emissions over Nights, Days, and Seasons”), 3-D Winds missions studying tropospheric wind conditions using space-based laser systems, ozone detection missions, EDL (“Entry, Descent, and Landing”) missions for the exploration of Mars and other celestial bodies, Automatic Rendezvous and Docking, and space communications. Such missions would benefit from an increase in available laser efficiency and output power, along with corresponding decreases in the mass and size of the laser device. There are also many aircraft applications such as air data (air speed and direction) measurements and detection of clear air turbulence and wind shears that also require lasers of small size. Therefore, there remains a need for a compact, low mass/high-energy laser system for use in the types of specialized applications noted above.